The present invention relates to a method for maintaining the quantity of NO that is converted to NO2 under different conditions in an oxidation catalytic converter arranged in a motor vehicle comprising an internal combustion engine, which in operation emits exhaust gases to an exhaust gas aftertreatment system comprising said oxidation catalytic converter. The method is intended for application in situations when a predetermined quantity of hydrocarbons is delivered to the exhaust gas aftertreatment system or the engine over at least one predetermined time interval as a temperature-boosting measure for heating up or regenerating one or more different exhaust gas aftertreatment units in the exhaust gas aftertreatment system.
The present invention also relates to an engine-driven vehicle comprising an internal combustion engine, which in operation emits exhaust gases to an exhaust gas aftertreatment system comprising an oxidation catalytic converter for oxidizing NO to NO2, a control unit and injection elements which are designed to inject hydrocarbons into the exhaust gas aftertreatment system, the control unit being designed to inject a predetermined quantity of hydrocarbons via the injection elements over at least one predetermined time interval.
The present invention also relates to a computer program for performing such a method with the aid of a computer.
The statutory requirements relating to diesel engines have been tightened up and will continue to become more stringent, particularly in relation to emissions of nitrogen oxide pollutants and particulate emissions.
The quantity of nitrogen oxides formed by the combustion of fuel in an engine cylinder depends on the combustion temperature. Higher temperatures lead to a greater proportion of the atmospheric nitrogen being converted into nitrogen oxides. The catalytic converters used on diesel engines and other engines that operate with excess air are only oxidizing. Since the exhaust gases contain oxygen it is difficult to be highly selective in reducing the nitrogen oxides. In addition to nitrogen oxides, other unwanted emissions formed in the combustion process include carbon monoxide (CO), hydrocarbons (HC) and particulates, primarily in the form of soot (C).
A known method for reducing the quantity of nitrogen oxides, which is based on exhaust gas aftertreatment, is the LNA NOx trap, (Lean NOx Adsorber). The LNA may also be referred to as the LNT (Lean NOx Trap). In this technique NO is first oxidized in an oxidation catalytic converter to form NO2, following which NO2 is stored in the trap in the form of nitrates. The storage of NO2 occurs when the engine is operating with excess oxygen. The NOx trap is then regenerated intermittently at predetermined intervals by allowing the engine to run with deficient oxygen, that is to say with an extra addition of hydrocarbon (usually the vehicle fuel) and/or reduced air flow, which heats up the nitrates and reduces the nitrogen dioxide NO2 caught in the NOx trap to nitrogen N2 and water H2O, see EP 1245817, U.S. Pat. Nos. 5,473,887 or 6,718,757, for example. Both the storage and the regeneration require a sufficiently high temperature in the NOx trap (slightly in excess of 2000 C for storage and approximately 3000 C for regeneration). At low engine loads (for example in town driving or in the case of an unloaded truck) the exhaust gas temperature will not be sufficient to keep the NOx trap at the necessary temperature. One way of boosting the temperature to a suitable level is then to inject hydrocarbons into the exhaust gas that is being burned catalytically in the exhaust gas aftertreatment system, so that the right temperature is reached. The hydrocarbons have a negative effect on the useful NO2 formation, with the result that the overall conversion of nitrogen oxides in the exhaust system diminishes during the warm-up phase.
Hydrocarbon can be supplied through an additional injection (post-injection) with open engine exhaust valve or via an injector arranged on the exhaust pipe.
Another known method of exhaust gas aftertreatment to which the formation of NO2 by means of an oxidation catalytic converter is central is CRT™ (Continuously Regenerating Trap). In this case particulates, that is to say soot and sulfur pollutant emissions, for example, are collected in a trap in which the soot can be converted to carbon dioxide CO2. NO2 here acts as oxidizing agent for the conversion of the particulates. In order for combustion of the soot to occur with the aid of NO2, the temperature of the aftertreatment system needs to be in excess of 2500 C. Here also, the temperature in the exhaust gas aftertreatment system can be boosted to a suitable level by the addition of hydrocarbons, which are burned in the catalytic converter, which nevertheless has a negative effect on the useful NO2 formation, so that the overall conversion of particulates in the exhaust gas aftertreatment system diminishes.
Other known exhaust gas aftertreatment methods to which the formation of NO2 is central are:                LNC (Lean NOx Catalyst), which continuously reduces nitrogen oxides under oxygen-rich conditions,        precious metal-coated particle filters,        urea or ammonia-based SCR (Selective Catalyst Reduction) for NOx reduction, see U.S. Pat. No. 5,540,047, for example,        hydrocarbon-based (HC-based) SCR (Selective Catalyst Reduction)        
It is desirable to maintain the level of useful NO2 formation in the oxidation catalytic converter for as long as possible, so that the overall efficiency of the exhaust gas aftertreatment system increases, that is to say that the exhaust gas aftertreatment system is capable of further reducing the emitted quantity of NOx and particulates, for example.
The method according to the invention comprises a method for maintaining the oxidation of NO to NO2 in an oxidation catalytic converter arranged in a vehicle comprising an internal combustion engine, which in operation emits exhaust gases to an exhaust gas aftertreatment system comprising said oxidation catalytic converter, a predetermined quantity of hydrocarbons being delivered to the exhaust gas aftertreatment system or the engine over at least one predetermined time interval. The invention is characterized in that said quantity of hydrocarbons is delivered to the exhaust gas aftertreatment system or the engine through a fragmented injection with a fragmentation frequency in the interval 0.01 to 0.5 Hz and an injection period in the interval 1 to 10 seconds, in which a shorter injection period is used at a higher fragmentation frequency, with the aim of largely maintaining an NO2 production in the exhaust gas aftertreatment system (2, 22, 32) that prevailed immediately prior to said time interval for the hydrocarbon injection.
One advantage of the method according to the invention is that by selecting the correct fragmentation frequency and injection interval for different conditions when warming up or regenerating a catalytic converter or particle filter, arranged in the exhaust gas aftertreatment system, an earlier conversion of NO to NO2 is maintained in an oxidation catalytic converter for as long as possible, even during a warm-up or regeneration process, that is to say when warming up or regeneration is taking place through the injection of hydrocarbon into the exhaust gas aftertreatment system. By maintaining the earlier conversion of NO to NO2, the total quantity of NOx and particulates in the exhaust gases leaving the exhaust gas aftertreatment system will be further reduced. In the case of an exhaust gas aftertreatment system with LNA, the conversion of NO to NO2 in an oxidation catalytic converter means that the conversion of NO to NO2 is maximized, whereas if the exhaust gas aftertreatment system instead comprises an SCR, the conversion of NO to NO2 means that a 50% oxidation is attained.
A further advantage of the invention is that the hydrocarbon injected will be used more efficiently, which affords a reduced consumption of hydrocarbon (approximately 5%).
The invention also encompasses an arrangement in the form of an engine-driven vehicle having an exhaust gas aftertreatment system in which the conversion of NO to NO2 in an oxidation catalytic converter is maintained by the continuous delivery of hydrocarbon to the exhaust gas aftertreatment system.
The same advantages are obtained by the arrangement according to the invention as by the method according to the invention.